Let me explain using two brief and arbitrary examples:
Rom 5:1: Then being justified by faith, we have peace with God through our Lord Jesus Christ.
Rom 8:16-17 The Spirit Himself witnesses with our spirit that we are children of God. And if children, also heirs; truly heirs of God, and joint-heirs of Christ, if indeed we suffer together, that we may also be glorified together.
I have heard Christians simply assume that they are part of the "us" group being discussed in verses like these, and I think I've worked out why. Whatever they think it is that makes them "Christians" (often something like believing, trusting and accepting Jesus and his atoning death for our sins), they assume the apostolic church had the same view. They assume that the group of Jesus-followers Paul means by "we" and "us" and "our" was also defined by similar beliefs or behaviors - even if it wasn't. So they think they really are part of the same "us" group - even if they aren't. And so they think statements that applied to the early Jesus-groups also apply to them - even if they don't.
As a result, I have observed Christians often experience a kind of dissonance, a tension in their minds. They read and earnestly feel certain things apply to them, and yet they don't always seem to experience these things in their real lives. Their beliefs and observations don't line up. Yet often, Christians seem to deal with this by saying "well it must be true because it's in the bible, so I'll just believe it" rather than considering whether or not they are included in the group referred to. They muster up "faith" to believe what doesn't seem to be true rather than face the frightening thought that they might not be in the "us" group. "Faith" becomes the rug under which reality is swept.
I think a large number of Christians are blinded by such "faith". Instead of realising that they really aren't authentically experiencing the sort of life Jesus was on about, they remain stagnant and unaware that they're missing out. They think they're experiencing authentic Christianity and so they don't search to discover it and truly experience it. This, I think, is sad.